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Widespread exploitation of horizontal drilling and hydraulic fracturing techniques has made it possible to extract vast amounts of natural gas previously locked in shale formations throughout the United States. This relatively recent development has likely changed the domestic energy landscape for decades to come. Figure X illustrates just how quickly shale gas production and proven reserves have ramped up in recent years; Figure X shows the latest projections for domestic gas production through the year XXXX from the Energy Information Administration (EIA, the federal entity responsible for tracking energy production and consumption in the U.S. and worldwide).

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Shale Gas Compositions

The production and transport of shale gas is posing some unique challenges for the natural gas industry. The existing infrastructure for producing and transporting "conventional" natural gas, which has been the norm for nearly the past XXX years, has been optimized to efficiently process and deliver gas that meets end-user requirements for heating value, hydrocarbon dew point, and contaminant content. Produced shale gases observed to date have shown a broad variation in compositional makeup, with some having wider component ranges, a wider span of minimum and maximum heating values, and higher levels of water vapor and other substances than pipeline tariffs or purchase contracts typically allow.

Table X presents some example shale gas compositions from a 2009 study. (X) These compositions were taken from shale plays that supply gas to the transmission network and end-users, including the Antrim, Barnett, Fayetteville, Haynesville, Marcellus, and New Albany plays. Table l also lists the historical mean values of these same components in pipeline-quality gases around the U.S. (X) (Note that none of the shale gas compositions listed in Table X were shown to contain butane or pentane (highlighted in yellow in Table X).

It is not known if these components were truly absent from the sample gases or, instead, the analyses were incomplete.) Some produced shale gases contain significantly higher amounts of ethane than the historical average of transmission-grade gas, while others have shown levels of diluents or hexanes and heavier hydrocarbons above historical averages. Not only are shale gases different from historical transmission-quality gases, they vary from one formation to another, and even within the same formation. Significant differences in levels of ethane, propane, hexane and heavier components, and diluents (primarily C[O.sub.X] and nitrogen) have been seen among the various shale formations. These, in turn, result in significant differences in the heating value, Wobbe number, and other parameters that guide end-use applications of natural gas.

Because of these variations in gas composition, each shale gas formation can have unique processing requirements for the produced shale gas to be marketable. Ethane can be removed by cryogenic extraction while carbon dioxide can be removed through a scrubbing process. However, it is not always necessary (or practical) to process shale gas to make its composition identical to "conventional" transmission-quality gases. Instead, the gas should be interchangeable with other sources of natural gas now provided to end-users. The interchangeability of shale gas with conventional gases is crucial to its acceptability and eventual widespread use in the U.S.

Concerns About Interchangeability

For most of the last decade, prior to the shale gas boom, natural gas demand in the U.S. exceeded conventional gas production and it appeared inevitable that the U.S. would be importing a significant quantity of LNG to meet future demand. With that prospect, there was growing concern that differences between the compositional makeup of imported LNG and conventionally produced natural gas would result in serious combustion efficiency and emissions problems at the burner tip. There was so much concern that the Federal Energy Regulatory Commission (FERC) asked the Natural Gas Council and other interested parties to make recommendations on how to deal with the anticipated problem.

The parties approached by FERC (collectively known as NGC+) subsequently developed a White Paper (X) with interim recommendations for addressing limits on natural gas quality parameters to help ensure continued clean and efficient combustion of natural gas at the burner tip. Although the LNG import era is apparently waning, the gas composition challenges posed by an increasing amount of LNG in the gas supply stream have some similarities to those posed by shale gas.

The concept of natural gas "interchangeability," discussed extensively in the NGC+ White Paper, is what helps ensure clean and efficient combustion at the burner tip, regardless of the compositional makeup of the sources of gas that constitute the supply stream. The NGC+ White Paper defines interchangeability as "the ability to substitute one gaseous fuel for another in a combustion application without materially changing operational safety, efficiency, and performance or materially increasing air pollutant emissions."

Interchangeability is described in technically based quantitative measures, such as indices, that predict combustion efficiency, emissions, flame stability, and gas-fired equipment performance. These indices have demonstrated broad application to end uses, and can be applied without discrimination of either end-users or individual suppliers. The White Paper recommended interim guidelines on four indices of interest:

X. The Wobbe number for a given area (an index related to energy flow rate to a combustion device) should remain within [+ or -]X% of the local historical average or established target gas value, subject to a maximum limit of X,XXX Btu/scf.